Ultrafast long-distance optical transmission systems at 40 Gbit/s and 100 Gbit/s or faster are becoming widespread succeeding optical transmission systems at 2.5 Gbit/s and 10 Gbit/s. An optical phase shift keying scheme and a digital coherent reception scheme are the most likely to be adopted in the ultrafast long-distance optical transmission system. Herein, the optical phase shift keying scheme has excellent characteristics required in long-distance optical fiber transmission, such as optical signal-to-noise ratio resistant characteristics, chromatic dispersion resistant characteristics, and polarization mode dispersion resistant characteristics. In the digital coherent reception scheme, coherent detection on a reception side and a digital signal processing technology are combined together.
A binary phase shift keying (BPSK) scheme and a quadrature phase shift keying (QPSK) scheme are particularly the most promising among phase modulation schemes in terms of balance of a transmission characteristic, easiness of fabrication, and a cost. Furthermore, consideration is also underway regarding a quadrature amplitude modulation (QAM) scheme having excellent spectral efficiency of optical frequency usage in order to increase a transmission capacity without increasing an optical frequency bandwidth being used. Optical multi-level phase and amplitude modulation schemes such as 16 QAM and 64 QAM respectively using a quadrature drive signal and an octal drive signal have been actively researched and developed. PTLs 1 to 5, for example, disclose these optical transmission schemes in detail.
PTL 1 discloses an optical modulation device that superimposes a low-frequency signal on a bias voltage, extracts the superimposed low-frequency signal from a modulation signal output from a modulator, and controls the bias voltage of the modulator, and that also superimposes a low-frequency signal on an amplitude control signal provided to a drive circuit, extracts the superimposed low-frequency signal from a modulation signal, and controls an amplitude of the modulation signal.